This invention relates to an electrochromic assembly and more specifically to electrochromic mirrors and windows.
Electrochromic mirrors and windows have historically been constructed with glass elements. Electrochromic mirrors that are typical of modern day automatic rearview mirrors for motor vehicles are disclosed in U.S. Pat. No. 4,902,108, entitled "SINGLE-COMPARTMENT, SELF-ERASING, SOLUTION-PHASE ELECTROCHROMIC DEVICES SOLUTIONS FOR USE THEREIN, AND USES THEREOF," issued Feb. 20, 1990, to H. J. Byker; Canadian Patent No. 1,300,945, entitled "AUTOMATIC REARVIEW MIRROR SYSTEM FOR AUTOMOTIVE VEHICLES," issued May 19, 1992, to J. H. Bechtel et al.; U.S. Pat. No. 5,128,799, entitled "VARIABLE REFLECTANCE MOTOR VEHICLE MIRROR," issued Jul. 7, 1992, to H. J. Byker; U.S. Pat. No. 5,202,787, entitled "ELECTRO-OPTIC DEVICE," issued Apr. 13, 1993, to H. J. Byker et al.; U.S. Pat. No. 5,204,778, entitled "CONTROL SYSTEM FOR AUTOMATIC REARVIEW MIRRORS," issued Apr. 20, 1993, to J. H. Bechtel; U.S. Pat. No. 5,278,693, entitled "TINTED SOLUTION-PHASE ELECTROCHROMIC MIRRORS," issued Jan. 11, 1994, to D.A. Theiste et al.; U.S. Pat. No. 5,280,380, entitled "UV-STABILIZED COMPOSITIONS AND METHODS," issued Jan. 18, 1994, to H. J. Byker; U.S. Pat. No. 5,282,077, entitled "VARIABLE REFLECTANCE MIRROR," issued Jan. 25, 1994, to H. J. Byker; U.S. Pat. No. 5,294,376, entitled "BIPYRIDINIUM SALT SOLUTIONS," issued Mar. 15, 1994, to H. J. Byker; U.S. Pat. No. 5,336,448, entitled "ELECTROCHROMIC DEVICES WITH BIPYRIDINIUM SALT SOLUTIONS," issued Aug. 9, 1994, to H. J. Byker; U.S. Pat. No. 5,434,407, entitled "AUTOMATIC REARVIEW MIRROR INCORPORATING LIGHT PIPE," issued Jan. 18, 1995, to F. T. Bauer et al.; U.S. Pat. No. 5,448,397, entitled "OUTSIDE AUTOMATIC REARVIEW MIRROR FOR AUTOMOTIVE VEHICLES," issued Sep. 5, 1995, to W. L. Tonar; and U.S. Pat. No. 5,451,822, entitled "ELECTRONIC CONTROL SYSTEM," issued Sep. 19, 1995, to J. H. Bechtel et al., each of which patents is assigned to the assignee of the present invention and the disclosures of each of which are hereby incorporated herein by reference.
In most cases, when the electrochromic medium (which functions as the media of variable transmittance in mirrors and windows) is electrically energized, it begins to absorb light. The more light the electrochromic medium absorbs, the darker the window or mirror appears. When the electrical voltage is decreased to zero, the mirror or window returns to its clear high reflectance state or fully transmissive state, respectively. In most commercially-produced electrochromic mirrors, the electrochromic medium sandwiched and sealed between the two glass elements is comprised of a solution-phase and a self-erasing system of electrochromic materials. Other electrochromic media may be utilized. Those media include an approach wherein a tungsten oxide electrochromic layer is coated on one electrode with a solution containing a redox active material to provide the counter electrode reaction.
When operated automatically, the electrochromic assembly of the indicated character, when implemented as a window, can incorporate light-sensing electronic circuitry, which is effective to change the electrochromic assembly to the absorbing modes when high ambient light levels are detected. The sandwiched electrochromic medium being activated and the transmissivity of the electrochromic assembly can be changed in proportion to the level of light that is detected. As the light level decreases, the electrochromic assembly automatically returns to its normal high transmission state. Alternatively, to reduce the air conditioning load on a building, the windows on the sides of the building exposed to direct sunlight can darken automatically and return to their normal high transmission state when in the shade.
When the electrochromic assembly is implemented as a mirror, the conductive layers on both the front glass element and the rear glass element are connected to electronic circuitry, which is effective to electrically energize the electrochromic medium to allow the mirror to relieve glare from the mirror.
The electrochromic medium fills a sealed chamber that has been defined by a ransparent front glass element, a peripheral edge seal and a rear glass element (that may include a reflective layer). Conductive layers have been provided on the inside of the front and rear glass elements. The conductive layer on the front glass element has been transparent while the conductive layer on the rear glass element may be transparent or may be semi-transparent or opaque and may also have reflective characteristics and function as a reflective layer for a mirror assembly.
Non-planar electrochromic mirrors have historically been manufactured by heating a planar glass substrate and bending it into the desired shape. Such non-planar mirrors are commonly used for some outside rearview mirror assemblies. Problems arise, however, in electrochromic mirror assemblies using two glass elements, which must both be bent into conforming shapes. If the front surface of the rear element does not conform identically to the rear surface of the front glass element, the spacing between the elements will not be uniform. This leads to non-uniform coloration of the electrochromic device in its low reflectance state. Additionally, such variations in curvature between the front and rear elements will introduce substantial image distortion into the reflected image. This problem becomes worse when the reflective layer of the mirror assembly is provided on the rear surface of the rear glass element.
To reduce the weight of the two glass element mirror assembly, the glass elements have been made using thinner glass substrates. Unfortunately, as the thickness of the glass is decreased, the individual glass elements become more fragile and flexible and are more difficult to bend accurately. Thin glass is also more difficult to bend accurately using conventional glass bending techniques.
It is therefore difficult to produce a commercially desirable non-planar electrochromic mirror that has two thin glass elements because each thin glass element will be much more likely to flex, warp, bow and/or shatter. Properties of a solution-phase electrochromic device, such as coloring and clearing times and optical density when colored, are dependent on the thickness of the electrochromic layer (e.g., the spacing between the two glass elements). Maintaining uniform spacing is necessary to maintain uniform appearance. The spacing between thin glass elements can be easily changed even after device manufacture by applying subtle pressure on one of the glass elements. This creates an undesirable non-uniformity in the appearance of the device.
While some prior art patents have suggested replacing a glass element with an element composed of a polymer, no electrochromic mirrors or windows incorporating a plastic element and containing a solution-phase electrochromic material have been commercially available. This is due, in part, to the fact that optical plastics have historically degraded with long term exposure to electrochromic materials, especially solution based electrochromic materials. For example, many optical quality lenses have been constructed of various transparent plastics (e.g., CR39, acrylics and various polycarbonates). However, such transparent plastics deteriorate when exposed to typical electrochromic solvents (e.g., propylene carbonate, ethylene carbonate, acrylonitrile, acetonitrile, dimethylformamide and dimethylsulfoxide) currently used in solvent-phase electrochromic assemblies. The vulnerable plastic could be overcoated with a protective layer but it is very difficult to produce guaranteed pin hole or defect free coatings in mass production.